1. Field of the Invention
The present invention relates to a write once optical recording medium.
2. Description of the Related Art
Write once optical recording mediums, for example, write once optical discs using organic dye as recording material constituting their recording layers are widely used as is known well in a write once CD (Compact Disc) and DVD (Digital Versatile Disc), so-called CD-R (Compact Disc-Recordable) and DVD-R (Digital Versatile Disc-Recordable).
On the other hand, various kinds of recording mediums using inorganic recording materials have been proposed so far (see cited patent reference 1, for example).
However, at present, a recording medium using an organic material as a recording material (hereinafter simply referred to as an “organic recording medium”) is widely used as compared with a recording medium using an inorganic material as a recording material (hereinafter simply referred to as an “inorganic recording medium”.
The reason for this will be described below. That is, when the recording film is made of the inorganic material, freedom of reflectance is small so that the inorganic recording medium cannot be made compatible with a ROM (read-only memory). Further, when it is intended to improve recording characteristics and durability of the inorganic recording medium, the recording film should be composed of a multilayer film and therefore investment in plant and equipment should be required for its manufacturing system, for example, a sputtering system. As a consequence, a cost of the inorganic recording medium is unavoidably increased as compared with the case in which the recording film is made of an organic material.
On the other hand, in the write once optical recording medium using the organic material as its recording material, its recording layer can be deposited by a spin-coat method and it is sufficient that a reflecting film may be deposited on the above-mentioned recording layer. Hence, its manufacturing method is simple and facilities and a cost of its manufacturing system are inexpensive.
On the other hand, density of optical disc has been increased so far by shortening a wavelength of light from mainly a light source and by a numerical aperture (N.A.) of an objective lens. Since a blue semiconductor laser having a wavelength near a short wavelength of 400 nm has been put into practice at present, development of organic dye suitable for use as a light source with such a wavelength becomes necessary.
However, in the organic dye which can satisfy optical characteristics relative to light of a short wavelength, a size of a dye molecule tends to be reduced and freedom in molecule designing is decreased. As a consequence, the organic dye which can be applied to the spin-coat method and which can be satisfactorily recorded by a blue light source with the above-mentioned short wavelength are not yet developed to the extent that it can be put into practice.
Also, in a disc, what might be called a “Blu-ray Disc”, which is now commercially available as the standards of the next-generation optical disc, since its numerical aperture (N.A.) is 0.85, a permissible degree of a disc skew is small so that recording and reproducing blue laser light is irradiated on the recording layer from the side of a light transmission layer having a film thickness of 0.1 mm formed on the recording layer.
Then, in the Blu-ray Disc, as described above, in these circumstances in which light is introduced into the recording layer from the side of the light transmission layer, a land (so-called on-groove in the Blu-ray Disc) is standardized as a recording track.
However, when the organic dye film is deposited by a spin-coat method, a groove is fundamentally deposited so as to become thicker than a land.
Accordingly, in the case of the optical recording medium in which the organic dye film deposited by the spin-coat method is used as the recording film and which is advantageous from a standpoint of characteristics, it is desired that groove (so-called in-groove in the Blu-ray Disc) recording with a large film thickness should be used. In this case, a polarity becomes opposite to that of the ordinary case.
Therefore, it is desired that information should be recorded on the land in the recording medium deposited by the spin-coat method. In this case, as described above, when the recording medium is deposited by the spin-coat method, since an organic dye film having a large film thickness is deposited as the recording film within the groove, a problem arises, in which crosstalk between the lands of the recording portion is increased unavoidably.
As described above, in the case of the organic dye, there arises a problem, in which a dye film having a large film thickness is deposited ion the groove. On the other hand, according to the inorganic recording material that can be deposited by a sputtering method, land recording can be employed similarly as a phase-change material which is already commercially available and it is easy to take compatibility with respect to recording characteristics, servo characteristics or the like. Thus, it is to be understood that the recording medium made of the inorganic recording material is advantageous from a standpoint of characteristics.
Further, also in the optical systems of CD-R and DVD-R, it is frequently observed according to a similar reason that the inorganic recording medium will become superior to the organic recording medium.
However, in a recording medium in which a recording layer is formed by a sputtering method as in a phase-change recording medium, when there are many layers constituting the recording layer, a manufacturing process becomes complex and a problem of a cost arises. Accordingly, it is desired that the number of films constituting this write once optical recording medium should be less than three to four layers.    [Cited Patent Reference 1]: Official Gazette of Japanese laid-open patent application No. 11-144316